The present invention relates to clutches, and more particularly to clutches employed in vehicle drivelines.
Conventionally, vehicles employ clutches in the drive train to allow for shifting between gears and transferring torque. For example, in some automatic transmissions, when the gear shift lever is initially moved into the reverse position, a clutch within the transmission begins to engage. This particular clutch is employed to decelerate several internal transmission components to rest, which then cause the reversal of direction of rotation of other transmission components. The components may be decelerated from, for example, 250 RPM down to zero. The changes in momentum of these components undergoing these changes in speed create reaction forces, which are propagated through the drive-train, producing a harsh engagement. This harsh engagement is sometimes objectionable to vehicle occupants. The harsh engagement can most commonly occur in transmission shifts from park to reverse, reverse to drive, and drive to reverse.
A common type of clutch that is employed to decelerate the automatic transmission components is a multi-plate friction clutch. These clutches are filled with a fluid that helps to dissipate the heat and in the operation of the clutch. FIGS. 1-3 illustrate a prior art embodiment of a multi-plate friction clutch. This clutch assembly 18 includes a piston 20, a return spring 22, and a hydraulic feed port 24 employed for engaging and disengaging the clutch 18. The clutch assembly 18 also includes three separator plates 26 and a pressure plate 28, interleaved with three friction plates 30. There is also a wave spring 32 mounted behind the pressure plate 28. All of the plates and wave spring are mounted concentrically about the central axis of the clutch. Also mounted about the central axis of the clutch 18, and in engagement with the friction plates 30, is a rotating component 34, which may be a shaft connected to a gear set or other typical rotating components connected to a clutch. A stationary component or housing 36 is engaged with the separator plates 26 and pressure plate 28. When referring to a stationary or non-rotating component that the clutch engages in a transmission, this may also mean a component that does rotate, but at a different velocity than the rotating component engagement of the clutch, then, will force the two components to rotate at the same velocity, rather than bring the rotational component to a stop.
FIG. 2 illustrates the prior art clutch assembly 18 in the open (disengaged) position. One will note that there is a gap between the various plates 26, 28 and 30, so the rotating component 34 can generally rotated freely, relative to the fixed component 36, except for some drag due to the viscosity of the fluid. The clutch assembly""s braking capacity is controlled by a hydraulic circuit, not shown, that provides an increasing pressure to the piston 20. As the clutch is actuated, the piston 20 applies an increasing force to squeeze the rotating friction plates 30 up against the non-rotating separator plates 26. As the squeezing force increases, the braking torque increases, and the rotating component 34 decelerates more quickly. The squeezing pressure rises to a predetermined maximum level, which defines the final braking and holding capacity of the clutch assembly 18. In order to better control the rise in pressure, some employ an accumulator and an orifice (not shown) to better control the pressure rise rate, as well as, a wave spring 38 to soften the transients effects that occur near the complete clutch engagement.
For a clutch such as that shown in FIGS. 1-3 to be applied to the transmission discussed above, the time to decelerate the components to rest, and the rate at which the clutch gains braking capacity is critical to obtaining the proper operating characteristics. If the component deceleration is too quick, the engagement will feel harsh to the vehicle operator, and if done too slowly, the vehicle operator could begin to accelerate before the clutch is fully engaged, again causing a harsh feel. Neither the accumulator/orifice, nor the wave spring adequately account for both of these concerns. This is true because, for typical multi-plate friction clutches, all friction elements gain capacity simultaneously. For this clutch, then, in order to prevent objectionable torque disturbances during clutch engagement, there must be a slow fluid pressure rise rate. But this conventional clutch is too slow for many applications.
Furthermore, since these clutches are filled with oil, even if one slows the compression of the plates, they can still begin transferring torque at too great of a rate. This is because as the clutch is starting to close up, the plates are pressed close to one another, and viscous friction drag begins transferring the torque between plates even though there is not contact yet. So in this conventional clutch, it produces much more clutch capacity early in the piston closing stroke because of this viscous drag. Thus, in order to achieve an adequate overall clutch engagement, one must adequately control both the asperity torque (friction braking effect) and the hydrodynamic torque (viscous drag braking), which add together to cause the overall clutch torque (clutch braking).
Some have tried to make clutch engagement more gradual by employing waved friction plates. Some have tried by employing waved or Belleville washer cushion springs at the friction element interface in the clutch. Others have tried restrained separator plates, which provide friction plates with their own clearance, to reduce viscous drag. But all of these have proven to be inadequate to cause a progressive transfer of torque, but in a relatively short time, and be reliable and cost effective. Some have tried to prevent objectionable torque disturbances due to too quick of a clutch engagement by employing waved or Belleville springs at the clutch stack, or varying the orifices, or through adaptive fluid controls. However, each of these techniques also has its drawbacks.
Thus, it is desirable to have a relatively fast applying multi-plate friction clutch system, that minimizes torque disturbance transmitted to the vehicle drivetrain as the clutch engages. Further, there are other general applications of multi-plate friction clutches, where a soft engagement is desired, other than in automatic transmissions. In such other applications where a relatively quick, soft engagement is desired, the prior art clutches also may be inadequate to perform as needed.
In its embodiments, the present invention contemplates a clutch assembly for controlling the rotation between a rotating member and a non-rotating member. The clutch assembly includes a first set of frictional members, including at least two first frictional members; and a second set of frictional members, including at least three second frictional members interleaved with the first set of frictional members. One of the first set and the second set is rotationally fixed to the rotating member, and the other of the first and the second set is rotationally fixed to the non-rotating member. The clutch assembly also includes a compressor, having a series of positions ranging from an open position allowing for gaps between adjacent members of the first set and the second set of frictional members to a closed position that does not allow for gaps between adjacent members of the first set and the second set of frictional members. The second set of frictional members has at least one spring located at a periphery of at least one of the second frictional members, with the spring extending toward at least one of the other of the second frictional members for biasing the at least one of the second frictional members away from the other of the second frictional members.
The present invention further contemplates a method for controlling the rotation between a rotating member and a non-rotating member in a clutch assembly, the clutch assembly having a first set of frictional members rotationally coupled to the rotating member and a second set of frictional members interleaved with the first set and rotationally coupled to the non-rotating member, the method comprising the steps of: applying an initial pressure to squeeze the first set of frictional members and second set of frictional members toward each other; spring biasing at least one member of the second set of frictional members away from an adjacent one of the members of the second set of frictional members; applying a second range of pressure greater than the initial pressure to squeeze the first set of frictional members into contact with the second set of frictional members, for the members that are not spring biased away from one another; and applying a third range of pressure greater than the second range of pressure to squeeze the members in the second set of members that are spring biased away from one another into contact with the first set of frictional members mounted therebetween.
Accordingly, an object of the present invention is to provide a multi-plate clutch assembly which, during engagement, allows for tilting of the separator plates in order to progressively engage portions of the plate contact surfaces.
Another object of the present invention is to provide a multi-plate clutch which, during engagement, retards the application of some of the plates at the onset of clutch engagement in order to obtain sequential engagement of the plates.
An advantage of the present invention is that the clutch will have an improved engagement feel by minimizing the hydrodynamic torque gain and permitting a more gradual asperity torque gain within a clutch.
Another advantage of the present invention is that it provides a multi-plate friction clutch system that minimizes the affects of full face viscous drag, and also promotes each friction element to begin gaining capacity at staggered times from the other friction elements during the engagement process, in order to provide gentle, yet relatively quick, application of the clutch.
An added advantage of the present invention is that by employing springs between the separator plates, the separator plates and friction plates are held spaced apart better during open clutch operation, thereby potentially reducing the open clutch drag.